Chromatography is a set of techniques for separating a mixture into its constituents. For instance, in a liquid chromatography (LC) application, a solvent delivery system takes in and delivers a mixture of liquid solvents to an autosampler (also called an injection system or sample manager), where an injected sample awaits the arrival of this mobile phase. The mobile phase with the dissolved injected sample passes to a column. By passing the mixture through the column, the various components in the sample separate from each other at different rates and thus elute from the column at different times. A detector receives the separated components from the column and produces an output from which the identity and quantity of the analytes may be determined.
Some currently available nano/capillary LC injection systems use a fixed-volume sample injector or rely on the coordination of a pump and an injection valve to produce a variable volume of sample. Some typical HPLC (High Performance Liquid Chromatography) rotary injection valves utilize a fixed sample loop, in which sample is loaded by aspirating or pumping sample into the loop. Then the valve is repositioned to bring this loop online. These rotary HPLC injection valves typically have two positions: a load position, as illustrated by the rotary valve 10 in FIG. 1A; and an inject position, as illustrated in FIG. 1B.
In the load position, sample enters a sample-loading port 12. A conduit 14 of the valve 10 places the sample-loading port 12 in fluidic communication with a sample-loop port 16. The sample enters the sample loop 18, which extends from the sample-loop port 16 to a second sample-loop port 20. A second conduit 22 of the valve places the sample-loop egress port 20 in fluidic communication with a vent port 24. Also in the load position, carrier mobile phase enters a mobile phase ingress port 26 of the valve. A third conduit 28 of the valve fluidically connects the mobile phase ingress port 26 to a column port 30.
In the inject position, the valve 10 has rotated (here, counterclockwise) such that the conduit 14 now fluidically connects the sample-loop port 16 to the column port 30 and the conduit 28 fluidically connects the other sample-loop port 20 to the mobile phase ingress port 26. In this configuration, the mobile phase enters the sample loop to join the sample held within the sample loop 18, and both flow together out through the column port 30.
Such injection valves can be used in “full loop” mode by filling the entire sample loop 18 volume with sample, and in “partial loop” mode, where the sample is brought into only a portion of the loop. Using this “partial loop” mode, the amount of injected sample is variable. This “partial loop” mode increases the flexibility of these injection valves, because otherwise users would need to change the loop physically with every change of the desired injection volume. With the “partial loop” mode, the injection volume can be changed programmatically from run-to-run to accommodate different sample types.
Typical rotary injection valves use external loops formed of lengths of tubing that can be plumbed into the valve. This tubing allows the user to change the loop volume in those instances where, for example, the partial-loop mode does not accommodate the needed sample volume. These external loops work well for analytical-scale HPLC (i.e., 1 to 4.6 μm inner diameter (id) columns), where injection volumes are typically 5-100 μL. Capillary and nano-scale chromatography (75-300 μm column ids), however, typically require injection volumes of less than 1 μL, and often less than 100 nL. Tubing with a small enough id to transport these low volumes is difficult to find and the typical unswept volumes in the loop ports add undesirable chromatographic variance.
To address these low-volume requirements, rotary injection valves have been equipped with “internal loops”, where the sample loop is created as a groove on the rotor or on stator of the valve. While these loops are capable of generating small injection volumes, it generally would be extremely difficult to attempt a “partial loop” injection using a loop of this size, as an extremely precise sample loading system would need to be employed to precisely position a sample in this small volume. Therefore, this type of “fixed loop” nano/capillary scale injector has typically been used for full-loop injections only. As a result, users generally must replace the injector (or rotor/stator) when they wish to use a different injection amount.